Hematite Nanorod Arrays Research Articles

Precise control of TiO2 overlayer on hematite nanorod arrays by ALD for the photoelectrochemical water splitting

The short lifetime of electron–hole pairs and high electron–hole recombination rate at surface states significantly limit the practical applications of hematite (α-Fe2O3) photoanodes in photoelectrochemical (PEC) water splitting.

Computational fluid dynamics simulation of bubble hydrodynamics in water splitting: Effect of electrolyte inflow velocity and electrode morphology on cell performance

Efficient management of oxygen gas bubbles evolving at the photoanode is important for a good overall performance of a photoelectrochemical (PEC) cell. The process of formation, detachment, coalescence and rise affects the bubble residence time at or near the photoanode. A reduced bubble residence time promotes easier electrolyte access to the photoanode surface, thereby enhancing the oxygen evolution reaction (OER) kinetics. In this work, computational fluid dynamics (CFD) simulations using laminar flow and phase-field model in COMSOL Multiphysics are conducted to study the gas bubble hydrodynamics for different bubble configurations, sizes, and bubble gaps. CFD results predict enhanced bubble rise in the presence of forced convection, i.e., some electrolyte inflow velocity (u≠0m.s−1). Experimental results using hematite as the model electrode also show a higher photocurrent density (ca. 4.33 mA.cm−2 at 2 V vs. RHE) for u = 0.1 m.s−1, compared to that without flow (3.5 mA.cm−2 at 2 V vs. RHE). This indicates improved access of electrolyte at the electrode surface and faster OER. Further, CFD simulations for electrode morphology effects show a reduced bubble adhesion for hydrophilic hematite nanorod arrays compared to planar photoanode, due to capillary wicking. The continuous contact film detaches and results in a reduced blockage of active sites at the bubble base, thereby enhancing the PEC performance for the nanorod arrayed morphology. Here too, experimental results show a four-fold increase in photocurrent density (ca. 0.1 mA.cm−2 at 1.5 V vs. RHE) and a 210 mV cathodic shift in onset potential for nanorods, compared to planar electrodes (0.025 mA.cm−2 at 1.5 V vs. RHE). A significant reduction in charge transfer resistance is observed for the former, implying better OER kinetics. The results obtained help in the understanding of transient processes involving O2 bubble hydrodynamics and electrolyte usage efficiency in a PEC reactor.

Surface charge recombination matters for single-versus polycrystalline catalysts in the case study of hematite photoanodes

Charge recombination is a critical problem limiting the efficiency of catalysts for solar water splitting. Constructing both single- and polycrystalline structures have been proposed to tackle this issue, however, comparison of the two is mainly focused on the crystallinity and a comprehensive analysis of the underlying reasons is lacking. Herein, we show that the enhancement in water-oxidation activity of the single crystalline photoanode is dominated by the lower surface charge recombination as compared to the polycrystalline one, taking hematite nanorod arrays prepared by gas phase cation exchange to exclude the influence of shape as the model catalyst. In contrast, the unexpected lower bulk charge separation efficiency of single crystal than that of polycrystal indicates that increasing the crystallinity is actually not the major factor for improving bulk charge transport efficiency. Our study sheds light on the structure–property relationship of monocrystal versus polycrystal in the hematite photoelectrochemical cell, beneficial to design of high-efficient catalysts for solar energy conversion.

Regulating Sn self-doping and boosting solar water splitting performance of hematite nanorod arrays grown on fluorine-doped tin oxide via low-level Hf doping

Hematite (α-Fe2O3) nanorod arrays grown on fluorine-doped tin oxide (FTO) substrate exhibit outstanding solar water splitting efficiency, benefiting from Sn self-doping induced by high-temperature annealing. However, this Sn self-doping couldn’t be freely controlled without changing the optimized annealing conditions, which limits the further improvement of their photoelectrochemical (PEC) properties. Here, we report a facile hydrothermal synthesis with subsequent annealing approach to regulate the Sn diffusion via hafnium (Hf) doping as well as enhance the PEC performance of hematite photoanode. Upon increasing the Hf doping concentration, the Sn self-doping content was continuously suppressed. The very low doping-level of Hf (i.e., atomic Hf/Fe = 0.13 ∼ 1.54%) was sufficient for enhancing the electrical conductivity. The Hf-doped α-Fe2O3 with the optimized dopant concentration (Hf/Fe = 1.34%, denoted as 0.25-Hf-Fe2O3) showed a photocurrent density of 1.79 mA/cm2 at 1.23 V vs. RHE, 70% higher than that of the Sn self-doped one (Pristine-Fe2O3). The donor density of 0.25-Hf-Fe2O3 increased 2.5 times compared to Pristine-Fe2O3 while its space-charge resistance and charge transfer resistance declined by 40% and 22%, respectively, verifying Hf doping improves the charge carrier density and accelerates the charge transfer for solar water oxidation. We offered here a prospective dopant alternative for preparing superior hematite-based photoanode.

Fluorine-surface-modified tin-doped hematite nanorod array photoelectrodes with enhanced water oxidation activity

Severe charge recombination and sluggish water oxidation reaction (OER) kinetics significantly limit the practical application of hematite in photoelectrochemical (PEC) water-splitting devices. In this study, fluorine-surface-modified tin-doped hematite (F/Sn:Fe2O3) photoelectrodes have been fabricated by a hydrothermal method incorporated impregnation annealing process. The grown FeOOH nanorods coated with NH4F solution are annealed first at 550 °C in the vacuum to modify the hematite surface and then at 750 °C in argon to promote the diffusion of tin atoms from the fluorine-doped tin oxide (FTO) glass substrate to the hematite structure. The synergistic effect of F-modification and Sn-doping on Fe2O3 electrode considerably enhances its PEC water oxidation performance, resulting in the highest photocurrent density of 3.64 mA cm−2 at 1.23 V vs. reversible hydrogen electrode (RHE) under AM 1.5 G illumination. Both effects increase the carrier concentration in the photoelectrode, which improves its transport efficiency. Moreover, the surface-localized F species promote the OER process on the electrode surface and improve the charge separation at the electrode/electrolyte interface by increasing its hole supply rate under the increased electric field. This work may open a new avenue for fabricating novel photoelectrodes with high PEC water splitting efficiency.

Photoinduced electrochemiluminescence at nanostructured hematite electrodes

In this article, we study the photoinduced electrochemiluminescence (PECL) of a luminol analog (L-012: 8-amino- 5‑chloro-2,3-dihydro-7-phenyl-pyrido[3,4-d]pyridazine-1,4‑dione) on n-type hematite (α-Fe2O3) nanorod arrays deposited by hydrothermal synthesis on a transparent conducting oxide. We report that this material, when used as a photoanode, triggers PECL at a potential as low as -0.2 V vs Ag/AgCl and allows the amplification of the luminescence signal. The PECL concept should open up new perspectives based on light-addressable strategies in bioanalysis and imaging.

Effects of In Situ Co or Ni Doping on the Photoelectrochemical Performance of Hematite Nanorod Arrays

Co-doped and Ni-doped hematite (α-Fe2O3) nanorod arrays were prepared on fluorine-doped tin oxide (FTO) conductive glass via aqueous chemical growth, in which the doping and the formation of nanorods occurred simultaneously (i.e., in situ doping). These samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), ultraviolet (UV)–visible spectrophotometry, linear sweep voltammetry and Mott–Schottky (M–S) measurement. Results showed that the introduction of 5% Co or Ni into α-Fe2O3 (the molar ratio of dopant to Fe is 1:20) did not change its crystal phase, morphology, energy gap and flat band potential. Both the undoped and the doped α-Fe2O3 showed a direct band gap of 2.24 eV, an indirect band gap of 1.85 eV, and a flat band potential of −0.22 V vs. saturated calomel electrode (SCE). At an applied potential of 0.2 V vs. SCE, the Co-doped and the Ni-doped α-Fe2O3 exhibited a photocurrent of 1.28 mA/cm2 and 0.79 mA/cm2, respectively, which were 2.1 times and 1.3 times that of the undoped α-Fe2O3. After the Co or Ni doping, the charge carrier concentration increased from 1.65 × 1025 m−3 to 3.74 × 1025 m−3 and 2.50 × 1025 m−3, respectively. Therefore, the increase in the photocurrent of the doped α-Fe2O3 was likely attributed to their enhanced conductivity.

Length-dependent photo-electrochemical performance of vertically aligned hematite nanorods

Photo-electrochemical (PEC) cells have been widely studied as an eco-friendly method of producing hydrogen fuel. Among the various materials, hematite (α-Fe2O3) is one of the most promising candidates for PEC applications due to its chemical stability and visible-range bandgap. However, despite the aforementioned advantages, hematite-based PEC cells have suffered from an extremely short hole diffusion length and charge carrier lifetime, resulting in a solar-to-hydrogen efficiency far lower than the theoretical maximum. To overcome these drawbacks, we controlled the length of vertically aligned hematite nanorod (NRs) arrays on a fluorine-doped tin oxide substrate by adjusting the chemical concentrations of the precursor solutions. We confirmed that the PEC performance of the hematite NRs was strongly dependent on their length and showed an approximately inverse proportionality between the length of the hematite NRs and their photoactivity. In addition, the hematite NRs array, with an optimized length, was further modified by cobalt phosphate (Co-Pi) cocatalysts to enhance the water oxidation kinetics and showed 1.54 mA cm−2 of photocurrent at 1.23 V vs. RHE.

Dual‐Doped Hematite Nanorod Arrays on Carbon Cloth as a Robust and Flexible Sodium Anode

AbstractRechargeable batteries with flexibility can find tremendous applications in wearable and bendable electronics. One central mission for the advancement of such high‐performance batteries is the exploration of flexible anodes with electrochemical and mechanical robustness. Herein reported is a robust and flexible sodium‐ion anode based on self‐supported hematite nanoarray grown on carbon cloth. The ammonia treatment that results in dual doping of both nitrogen and low‐valent iron renders surface reactivity and electric conductivity to the material. The dual‐doped hematite arrays afford a robust activity for sodium storage, exhibiting reversible capacities of 895 and 382 mAh g−1 at current rates of 0.1 and 5 A g−1, respectively, or 615 and 356 mAh g−1 by removing the contribution of the substrate. They also sustain 85% of the initial capacity upon 200 cycles at 0.2 A g−1. To demonstrate the flexibility, full cells composed of a hematite array anode and Na3V2(PO4)3/C cathode are assembled. The cell is capable of affording an energy density of 201 Wh kg−1 and sustaining repeated bending without performance decay, demonstrating a significant potential in practical application.

Cobalt-Phosphate Catalysts with Reduced Bivalent Co-Ion States and Doped Nitrogen Atoms Playing as Active Sites for Facile Adsorption, Fast Charge Transfer, and Robust Stability in Photoelectrochemical Water Oxidation.

A cobalt-phosphate (Co-Pi) catalyst having octahedral CoO6 molecular units as reaction sites is a key component in photoelectrochemical (PEC) water oxidation systems, but its limited adsorption sites for oxygen-evolving intermediates (*OH, *OOH), slow charge transfer rates, and fast degradation of reaction sites are yet to be overcome. Here, we report that Co-Pi nanoparticles with low-coordinate Co ions and doped nitrogen atoms could be decorated on hematite nanorod arrays to form N-CoPi/hematite composites. Moreover, the local atomic configuration and bond distance studies show that trivalent Co3+ states are partially reduced through nitrogen radicals in the plasma to low-coordinate bivalent Co2+ states playing as the facile adsorption sites of oxygen-evolving intermediates due to the decreased activation barrier for water oxidation. Electron transport is also reinforced by nitrogen species due to the formation of hybridizing N 2p orbitals that give the acceptor levels in the bandgap. As a result, both the incident photon-to-electron conversion efficiency and the charge transfer resistance on N-CoPi/hematite outperform those on a bare hematite by about 3 fold. Furthermore, N-CoPi/hematite gives high activity retention over 90% after the long operation of water oxidation, in support of the reaction sites on N-CoPi not degrading during the successive water oxidation.

Conformally Coupling CoAl-Layered Double Hydroxides on Fluorine-Doped Hematite: Surface and Bulk Co-Modification for Enhanced Photoelectrochemical Water Oxidation.

Earth-abundant hematite is an attractive photoanode for photoelectrochemical water splitting, whereas the intrinsic properties of inferior charge transfer and slow water oxidation kinetics still hinder its application. In response, an integrated photoanode has been constructed with hematite nanorod arrays modified by fluorine anion doping and further decorated with amorphous CoAl-layered double hydroxides (CoAl-LDH). This novel CoAl-LDH/F-Fe2O3 photoanode exhibited an excellent photocurrent density of 2.46 mA cm-2 at 1.23 V versus reversible hydrogen electrode (VRHE), five times enhanced than that of pristine α-Fe2O3. Systematic investigations reveal that fluorine anion serving as a donor dopant dramatically enhances the density of charge carrier and reduces the resistance of hematite for rapid charge transfer. Furthermore, the cocatalyst of CoAl-LDH could effectively passivate the surface defects of F-Fe2O3 and facilitate the water oxidation kinetics through an alternative pathway of holes trapped by Co species. As a consequence, the charge separation efficiencies of the bulk and surface were improved to 32.6 and 81.8%, respectively, compared with those of α-Fe2O3 (9.7 and 31.7%). Our results demonstrate that the dual modification of the bulk and surface is an attractive maneuver to ameliorate the water oxidation activity of hematite.

Effects of Ti Doping on Hematite Photoanodes: More Surface States.

Ti doped hematite photoanodes have been intensively investigated due to their excellent activity for photoelectrochemical water oxidation. However, little attention has been paid to the doping effect on the photocurrent onset potential of hematite and the underlying mechanism. In this paper, Ti doped hematite nanorod arrays were successfully prepared through a facile treatment of hematite with TiCl₃ solution. The photocurrent of the Ti doped hematite photoanode increases by three times, and its onset potential shifts more positively as compared with that of the undoped one. Electrochemical analyses were employed to unravel the mechanism of anodic shift of the onset potential. Cyclic voltammograms and electrochemical impedance spectra confirmed that more surface states were formed in Ti doped hematite than the undoped one. As a result, lower activity towards oxygen evolution reaction (OER) and increased electron-hole recombination after light on/off in low potential region were observed in Ti doped hematite. It is concluded that these doping induced surface states may be a hindrance to charge transfer and the onset potential of Ti doped hematite shifts anodically.

Selective doping of titanium into double layered hematite nanorod arrays for improved photoelectrochemical water splitting

Element doping is effective to improve the photoelectrochemical (PEC) performance of photoelectrodes, as it can increase the carrier density and then enhance electrical conductivity for efficient charge transfer. In this study, titanium (Ti) was selectively doped into the bottom and/or top layer of the double layered hematite (α-Fe2O3/α-Fe2O3) nanorod arrays grown on conductive transparent substrate (F:SnO2, FTO) via a two-step hydrothermal method to optimize the electron donor distribution and improve the charge separation efficiency for a remarkable enhancement in PEC water splitting. It was demonstrated that, by selectively doping Ti into the bottom layer, the obtained FTO/α-Fe2O3:Ti/α-Fe2O3 nanorod photoanode showed the highest PEC performance for water splitting, with photocurrent density reaching 1.69 mA/cm2 at 1.9 V vs. RHE under AM 1.5G illumination, which was 4.3 times that of undoped α-Fe2O3/α-Fe2O3 nanorod film (0.39 mA/cm2) and even much higher than the top layer and double layer doped α-Fe2O3 nanorod films (FTO/α-Fe2O3/α-Fe2O3:Ti and FTO/α-Fe2O3:Ti/α-Fe2O3:Ti). By introducing the Ti electron donor dopants into α-Fe2O3, the electron density will be increased in the α-Fe2O3:Ti layer, raising the Fermi level. For the FTO/α-Fe2O3:Ti/α-Fe2O3 nanorod film, the band realignment will create a terraced band structure and then build an internal electric field at the interface of the bottom and top layers. As a result, the photoexcited electrons and holes will transfer to the FTO substrate and the photoanode surface, respectively, as driven by the internal electric field. This study demonstrated an alternative approach to the design of novel photoanodes with improved PEC performances, by engineering the electron density distribution and the band structure for efficient charge carrier separation.

Impact of annealing temperature to the performance of hematite based humidity sensor

<span lang="EN-GB">In the present study, hematite (α-Fe<sub>2</sub>O<sub>3</sub>) nanorod structure were grown on fluorine doped tin oxide coated glass substrate via sonicated immersion approach with variation of annealing temperature (350˚C – 600˚C) in one-hour treatment. The impact of varying the temperature of annealing treatment on crystalline phase, structure morphology, optical properties and humidity sensing performance of hematite were examined. X-ray diffraction pattern disclosed a rhombohedral structure with α-phase diffraction peaks. The surface morphology images taken from field emission scanning electron microscopy revealed that the hematite nanorod arrays were grown uniformly in all samples and the average diameters of nanorods were measured in the ranges between 55 and 80 nm. Ultraviolet–visible spectroscopy measurement spectra show that all samples exhibited good optical properties. The hematite humidity sensor sample annealed at 400°C has demonstrated the highest sensitivity response (S=177.78) to humidity range between 40%RH to 90%RH.</span>

Hematite nanorod arrays top-decorated with an MIL-101 layer for photoelectrochemical water oxidation.

The construction of a nano-heterojunction photoanode is an attractive strategy for accelerated charge separation. Here, α-Fe2O3 nanorod arrays (Fe2O3 NAs) top-decorated with an MIL-101 layer are achieved via the CVD method for constructing one intimate contact between two layers. The systematical analysis in time-dependent deposition shows that the photoelectric response capability can be modulated by controlling the thickness of the MIL-101 layer as a function of CVD duration. An optimized thickness of 4-6 nm MIL-101 in the Fe2O3/MIL-101 photoanode presents the best PEC performance with the IPCE of 25% and a photocurrent density of 1 mA cm-2 at 1.3 V vs. RHE, about 2.5 times higher than that of pristine Fe2O3 NAs. The exciton lifetime and the electrochemical active surface area of the composited electrode are greatly improved by 14 and 1.3 times respectively compared to that of pristine Fe2O3. The notable advancement can be attributed to the fact that the top-decorated MOF layer can promote charge separation efficiency led by the built-in electric field and form a tight interface between Fe2O3 and MIL-101 with a favorable diffusion length for holes, as well as the fact that it can reduce surface defects and provide more reaction sites.

Protected Hematite Nanorod Arrays with Molecular Complex Co‐Catalyst for Efficient and Stable Photoelectrochemical Water Oxidation

Herein, a hybrid structure of hematite (α‐Fe2O3) nanorod arrays is designed, with surface catalyzed by Co molecular complex (Co(dca)2, dca: dicyanamide) and then protected by TiO2 thin overlayer, for efficient and stable photoelectrochemical (PEC) water oxidation. The obtained α‐Fe2O3/Co(dca)2/TiO2 hybrid nanorod arrays show much improved and stabilized PEC performance as compared to the pristine, and even the Co(dca)2 or TiO2 modified α‐Fe2O3, with a photocurrent density of 0.35 mA cm–2 obtained at 1.23 V vs. reversible hydrogen electrode (RHE), and an incident photon‐to‐current conversion efficiency (IPCE) reaching 16 % at 400 nm at 1.6 V vs. RHE. It has been demonstrated that the adsorbed Co(dca)2 molecular complex could effectively promote the interface charge transfer process and accelerate the water oxidation reaction kinetics, meanwhile the atomic layer deposited TiO2 overlayer could passivate the surface defects of α‐Fe2O3 and suppress the surface charge carrier recombination. Moreover, the TiO2 overlayer could effectively protect Co(dca)2 from detaching from the α‐Fe2O3 surface and thus stabilize the PEC activity for water oxidation reaction. The present study provides some available thoughts and methods for rational design of highly efficient photoelectrodes for water splitting from the perspective of the surface and interface engineered charge carrier transfer and water oxidation processes.

Charge carrier dynamics of Earth abundant co-catalyst (Ni) modified nanorod arrays for enhanced water splitting

The vertically oriented Ni (Ni0) decorated hematite nanorod array have been synthesized. The optimal Ni loading enhances photocurrent density > 4 folds at 1.23 V vs RHE and lowers the overpotential for driving the water oxidation, by ~70 mV. The Ni decorated hematite photoelectrode shows a longer decay time (~195 ps) and higher Stern-Volmer constant (5.25 M−1) than that of bare hematite photoelectrode (~156 ps and 3.89 M−1, respectively), implying an efficient charge separation and transfer across the hematite nanorod-Ni nanocrystal heterointerface. Such drastic improvement in the activity is due to the unified effects of enhanced charge separation and charge carrier densities, faster charge transfer kinetics, decreased width of depletion layer, and enhanced band bending – lead by the unison of the nanorod arrays morphology and Ni modification. These results suggest potential candidature of Earth abundant metallic Ni as a co-catalysts (-alternative to noble metals) for designing new photocatalysts.

Understanding the role of nanostructuring in photoelectrode performance for light-driven water splitting

The analysis of capacitance data for regular nanostructured photoelectrodes is revisited using a hematite nanorod array as an example. The effects of the cylindrical nanorod geometry on the capacitance-voltage behaviour are outlined, and the limiting case of complete depletion is discussed in terms of the residual geometric capacity at the base of the nanorods. Since nanorod arrays generally leave areas of the substrate exposed, it is necessary to consider the parallel capacitance associated with the fraction of uncovered surface. The sensitivity of the capacitance fitting to parameter variation is explored. The enhancement of external quantum efficiency (EQE) by nanostructuring is also discussed using hematite nanorod arrays as experimental examples. It is shown that, although very substantial EQE enhancement should be achieved by simple geometric effects, the performance of nanostructured hematite electrodes in the visible region of the spectrum is considerably lower than predicted if all charge carriers generated in the space charge region (SCR) were collected. Further analysis reveals that the internal quantum efficiency increases with photon energy, suggesting that the probability of generating free, rather than bound, electron-hole pairs in hematite depends on the excess energy hν - Egap.

Cobalt oxide and carbon modified hematite nanorod arrays for improved photoelectrochemical water splitting

Given the proper band gap, low cost and good stability, hematite (α-Fe2O3) has been considered as a promising candidate for photoelectrochemical (PEC) water splitting, however suffers from the sluggish surface water oxidation reaction kinetics. In this study, a simple dip-coating process was used to modify the surface of α-Fe2O3 nanorod arrays with cobalt oxide (CoOx) and carbon (C) for the improved PEC performance, with a photocurrent density at 1.6V (vs. reversible hydrogen electrode, RHE) increased from 0.10mA/cm2 for the pristine α-Fe2O3 to 0.37mA/cm2 for the CoOx/C modified α-Fe2O3 nanorods. As revealed by electrochemical analysis, thanks to the synergistic effect of CoOx and C, the PEC enhancement could be attributed to the enhanced charge transfer ability, decreased surface charge recombination, and accelerated water oxidation reaction kinetics. This study serves as a good example for improving PEC water splitting performance via a simple method.

Growth of bismuth oxyhalide nanoplates on self-standing TiO2 nanowire film exhibiting enhanced photoelectrochemical performances

We report the sequential modification of the free-standing TiO2 nanowires film with Ag nanoparticles and BiOX (X=Cl, Br, and I) nanoplates, for a purpose of achieving an efficient photoelectrochemical cells (PECs). In the resulting BiOX-Ag-TiO2 film, Ag nanoparticles densely coated on the broad and flat surfaces of TiO2 nanowires increase the conductivity of TiO2 by 107 and enable the film to propagate electrons rapidly. BiOX nanoplates obliquely grown on the Ag-coated TiO2 nanowires can move the absorption edge of the film into the region of visible light and endow the film with the anti-reflection property. Especially, when the BiOCl-Ag-TiO2, BiOBr-Ag-TiO2, and BiOI-Ag-TiO2 films are overlapped to form a laminated film, the anti-reflection response is further promoted. Besides the above optical and electronic metrics, the band alignment in the multi-component system favors the directed flow of photogenerated electrons from BiOX and TiO2 to Ag, thus efficiently blocking the recombination of the e−–h+ pairs. Therefore, under the illumination of simulated solar light, the photoanode fabricated by the laminated BiOX-Ag-TiO2 film could yield a small onset potential around 0.7V vs RHE and a large photocurrent approaching 10mA/cm2 at 1.2V vs RHE, which outperforms the state of the art photoanode based on IrO2 modified hematite nanorod arrays.